The subject invention relates to the design of a detector array for use in a broadband imaging spectrometer. The invention is broadly applicable to the field of optical metrology, particularly optical metrology tools for performing measurements of patterned thin films on semiconductor integrated circuits.
The use of thin film measurement technologies such as spectroscopic ellipsometry [SE], broadband reflectometry [BBR] and visible light reflectometry [VR] is well established. These technologies use a spectrometer to simultaneously gather information about the sample under test at different wavelengths. Examples in the prior art include U.S. Pat. Nos. 5,910,842 and 6,278,519 incorporated herein by reference. For optical wafer metrology the wavelength region of interest spans the VUV and NIR.
There are a number of optical metrology tools presently available for evaluating the characteristics of semiconductor wafers. One example of such a tool can be found in U.S. Pat. No. 6,278,519 cited above. In this tool, a broadband light source creates a probe beam that is focused onto the sample. The focused probe beam interacts with and reflects from the sample. The reflected probe beam is directed to a spectrometer for measuring variations in intensity versus wavelength. In this particular tool, the same spectrometer is used to obtain BBR data as well as SE data. Other prior art embodiments employ separate spectrometers for SE and BBR measurements. In either case, the spectrometer(s) will include an optical element, such as a grating, for dispersing the light as a function of wavelength. The dispersed light is directed onto a two-dimensional array of photo-detector elements that generate output signals corresponding to the intensity of the wavelength-dispersed light.
Ideally, in these measurements both the intensity and the angle of the incident light are substantially uniform over the illuminated spot. It is equally desirable to control the angular acceptance of the optical systems that collect the illumination after reflection from and interaction with the sample. This may be achieved through selection and location of apertures within the optical system.
It is well known to one of ordinary skill in the art that, in these instruments, the light reaching the detector includes contributions from at least two sources: the desired signal generated by light interacting with and specularly-reflecting from the sample, and an undesired spurious signal typically generated by light that is scattered from both the components of the spectrometer and sample being measured. The scattered light may adversely effect the BBR and SE measurements.
In mathematical terms the measured intensity at a given wavelength IT(xcexA) includes contributions due to reflection IR(xcexA) and scatter IT(xcexB)
IT(xcexA)=IR(xcexA)+IT(xcexB).xe2x80x83xe2x80x83(1)
As indicated in equation 1 the scattered radiation reaching a detector element configured to detect wavelength region xcexA may contain contributions from other wavelength ranges (i.e. xcexB in equation 1). In general, the ratio of the scattered to detected intensity depends on wavelength and varies with the condition and morphology of the sample being measured.
Prior art approaches have attempted to account for the scattered contribution and correct the measured data with varying degrees of sophistication. The simplest approach is to assume that the effect is small and account for xe2x80x9csample dependentxe2x80x9d variations by normalizing the measurements to those of a known sample. If these assumptions are invalid (i.e. scatter is non-negligible, or there are sample dependent effects) the data cannot be meaningfully corrected with this approach.
The next level of sophistication is to provide a detector to directly measure a fraction of the broadband radiation scattered out of the specularly-reflected beam. One prior art approach, illustrated in FIG. 1, locates a detector array 10, such that the wavelength dispersed, specularly-reflected beam illuminates a main measurement region 2 of array 10. Array 10 further includes a secondary region 4 located at one end of the array. Region 4 is located such that very little specularly-reflected light reaches region 4.
Array 10 is two-dimensional having a number of detecting elements across both its length (L) and its width (W). Region 4 is located at position to receive diffracted intensity corresponding to a wavelength region below the short-wavelength-cutoff (xcexshort) of the metrology system bandwidth (approximately 195 nm in typical commercial UV-VIS instruments). This location avoids potential contamination of the scattered signal by high-order diffracted light and insures that scattered light is the dominant source of the signals generated by the detector in region 4. If the scattered light illuminating region 4 is similar, in both intensity and spectral makeup, to the scattered light illuminating region 2, the measured data can be corrected by subtraction the signals from region 2 using the signals from region 4. In principle, the measurements made in region 4 can also be used to correct for any systematic variation in the background intensity along the width (W) of detector 10.
This approach to correcting the data is only valid if the intensity of the scattered light is constant along the length (L) of detector 10. This may be a reasonable approximation if the scattered intensity is truly diffuse so that any scattering element sprays light evenly across the entire array so that the scattered intensity measured in region 4 is substantially equivalent to that in region 2. In practice, the spread of the scattered light is not perfectly diffuse; typically the light entering region 4 is more heavily influenced by the intensity of the light striking the neighboring pixels of region 2 than by the light striking pixels at the other end of the detector. Consequently, this correction approach can also be inadequate for high accuracy measurements.
Accordingly, it would be desirable to provide a detection approach that more accurately measures the contribution of optical scatter to the measured, spectrally dispersed signal. This would enable superior correction of the raw data and thereby improve the measurement accuracy of the metrology system.
A method for correcting spectroscopic intensity measurements that includes correction of the data to remove spurious optical background signals is disclosed. An array detector, divided into two regions, simultaneously measures the signal and background intensity at the exit plane of the system spectrometer. The background measurements are used to correct the signal data. Correction protocols are described which permit removal (subtraction) of background components that vary in either one or two dimensions across the detector surface. This permits increased accuracy in spectroscopic intensity measurements.